Methods for controlling fluid delivery in a micro fuel cell system

ABSTRACT

Micro fuel cell systems whose performance is enhanced by an accurate fluid delivery system. The fluid delivery system improves reactant fluid provision to meet electrical output, while maintaining correct stoichiometries for chemical processing in a downstream reactor. The fluid delivery system includes a pressure source and a differential flow meter. The differential flow meter uses a flow restrictor and a sensor. The pressure source moves a fluid through the flow restrictor; the sensor detects differential pressure in the flow restrictor and outputs a signal that permits dynamic control of fluid flow, e.g., by controlling a pump.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under U.S.C. §120 and is a divisionalof U.S. patent application Ser. No. 11/193,303, filed Jul. 29, 2005 andentitled, “Method and System for Controlling Fluid Delivery in a FuelCell”, now U.S. Pat. No. 7,205,060, which claimed priority under 35U.S.C. §119(e) to: i) U.S. Provisional Patent Application No. 60/599,589filed on Aug. 6, 2004 entitled “Method and System for Controlling aMicro Fluid Delivery System”, ii) U.S. Provisional Patent ApplicationNo. 60/638,421 filed on Dec. 21, 2004 entitled “Micro Fuel CellArchitecture”, iii) U.S. Provisional Patent Application No. 60/649,638filed on Feb. 2, 2005 entitled “Heat Efficient Micro Fuel Cell System”,iv) U.S. Provisional Patent Application No. 60/677,424 filed on May 2,2005 entitled “Micro Fuel Cell Fuel Cartridge Apparatus”, and v) U.S.Provisional Patent Application No. 60/682,598 filed on May 18, 2005entitled “Fuel Storage Devices for Use with Micro Fuel Cells”; each ofthese patent applications is incorporated by reference in its entiretyfor all purposes.

BACKGROUND

The present invention relates to fuel cell and microfluidic technology.In particular, the invention relates to systems and methods ofcontrolling reactant fluids and pumps in micro fuel cell systems.

A fuel cell electrochemically combines hydrogen and oxygen to generateelectrical energy. Commercially available fuel cell systems are stillrestricted to large-scale applications, such as industrial sizegenerators for electrical power back up. Consumer electronics devicesand other portable electrical power applications currently rely onlithium ion and similar battery technologies. Portable fuel cell systemsoffer extended usage times over batteries and would be desirable, butare not yet available.

The air readily provides oxygen; hydrogen requires a dedicated source. Aportable storage device offers a replenishable hydrogen supply, and mayinclude an outlet that detachably couples to the fuel cell system andallows the storage device to be replaced when depleted. The hydrogensupply may include a direct hydrogen supply or a ‘reformed’ hydrogensupply. A direct hydrogen supply employs a pure source, such ascompressed hydrogen in a pressurized container, or a solid-hydrogenstorage system, such as a metal-based hydrogen storage device. Areformed fuel cell system processes a hydrogen fuel source to producehydrogen. The fuel source acts as a carrier for hydrogen, is manipulatedto separate hydrogen, and may include a hydrocarbon fuel, hydrogenbearing fuel stream, or other hydrogen fuel source such as ammonia.Liquid fuel sources offer high energy densities and the ability to bereadily stored and shipped.

One or more pumps move reactants into the fuel cell system. Portable andmicro fuel cell systems use low flow rates, typically less than 5milliliters per minute of methanol based fuels for example. Such lowflow complicates accurate control—yet the fuel cell system imposes tightdemands on hydrogen supply. At the least, the system must ensure thatthe hydrogen supply flow rate satisfies power generation in the fuelcell to meet electrical demand. The flow should also maintain correctstoichiometries for fuel processing in a reformed system; underflow maylead to an individual cell or two “going negative”, meaning that it canno longer sustain a reaction rate commensurate with the rest of thecells in a stack. Under these conditions, one or more cells in the stackmay be damaged and need replacement before the stack operates properlyagain.

Commercially available low flow rate pumps do not provide suitableaccuracy for portable fuel cell systems. Based on the foregoing,alternate techniques for reactant supply and fluid control in micro fuelcell systems are needed.

SUMMARY

The present invention relates to micro fuel cell systems whoseperformance is enhanced by an accurate fluid delivery system. The fluiddelivery system improves reactant fluid provision to meet electricaloutput, while maintaining correct stoichiometries for chemicalprocessing in a downstream reactor.

In one aspect, the present invention relates to a fuel cell system forgenerating electrical energy. The fuel cell system includes adifferential flow meter that includes a flow restriction and at leastone sensor configured to measure differential pressure in a fluidbetween two locations of the differential flow meter. The fuel cellsystem also includes a pressure source that moves the fluid to thedifferential flow meter before use of the fluid in the fuel cell system.The fuel cell system further includes a controller configured to converta signal output by the sensor to a command that affects flow rate of thefluid. The fuel cell system additionally includes a fuel cell configuredto receive oxygen and hydrogen and to generate electrical energy.

In another aspect, the present invention relates to a fuel cell system.The fuel cell system includes a storage device that stores a fuel sourceand a fuel processor that processes the fuel source to output hydrogen.The fuel cell system also includes a differential flow meter thatincludes a flow restriction and at least one sensor. The fuel cellsystem further includes a pressure source, a controller, and a fuelcell.

In yet another aspect, the present invention relates to a fuel cellsystem. The fuel cell system includes a pressure source configured tomove hydrogen at a flow rate that is less than about 60 milliliters perminute per watt output by the fuel cell.

In still another aspect, the present invention relates to a fuel cellsystem. The fuel cell system includes a pressure source configured tomove a liquid fuel source at a flow rate that is less than about 1milliliter per hour per watt output by the fuel cell.

In still another aspect, the present invention relates to a method forcontrolling fluid delivery in a fuel cell system. The method includesmoving a fluid at a predetermined flow rate. The method also includesflowing the fluid through a differential flow meter that includes a flowrestriction and at least one sensor configured to measure differentialpressure in the fluid between two locations of the differential flowmeter. The method further includes detecting differential pressure ofthe fluid between two locations of the differential flow meter while thefluid is in the differential flow meter. The method additionallyincludes determining flow rate of the fluid using the differentialpressure and changing the flow rate of the fluid.

These and other features of the present invention will be described inthe following description of the invention and associated figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a fuel cell package for producing electrical energyin accordance with one embodiment of the present invention.

FIG. 1B illustrates a fuel cell package including a fuel processor inaccordance with another embodiment of the present invention.

FIG. 1C illustrates schematic operation for the fuel cell package ofFIG. 1B in accordance with a specific embodiment of the presentinvention.

FIGS. 2A-2C illustrate fuel delivery systems in accordance with severalembodiments of the present invention.

FIG. 3 shows a process flow for controlling fluid delivery in a fuelcell system in accordance with one embodiment of the present invention.

FIG. 4A shows data that illustrates the controllability imparted by afluid delivery system of the present invention.

FIG. 4B shows flow data in a flow vs. frequency of a conventionaldiaphragm pump.

FIG. 5A illustrates an outer perspective view of a fuel cell package inaccordance with one embodiment of the present invention.

FIG. 5B shows a perspective view of internal components of a coplanarfuel cell package in accordance with a specific embodiment of thepresent invention.

FIG. 5C illustrates a perspective view of internal components for a fuelcell package in accordance with another specific embodiment of thepresent invention.

FIG. 6A illustrates a simplified cross sectional view of a fuel cellstack for use in the fuel cell of FIG. 1A in accordance with oneembodiment of the present invention.

FIG. 6B illustrates an outer top perspective view of a fuel cell stackand fuel cell in accordance with another embodiment of the presentinvention.

FIG. 6C illustrates an ion conductive membrane fuel cell (PEMFC)architecture for the fuel cell of FIG. 1A in accordance with oneembodiment of the present invention.

FIG. 6D illustrates a top perspective view of bi-polar plates inaccordance with one embodiment of the present invention.

FIG. 7A illustrates an outer top perspective view of a fuel processorused in the fuel cell system of FIG. 1A.

FIG. 7B illustrates a cross-sectional front view of a main component inthe fuel processor used in the fuel cell system of FIG. 1A taken througha mid-plane of fuel processor.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is described in detail with reference to a fewpreferred embodiments as illustrated in the accompanying drawings. Inthe following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process steps and/or structureshave not been described in detail in order to not unnecessarily obscurethe present invention.

Overview

Micro fuel cell systems generate dc voltage, which may be used in a widevariety of applications. For example, electrical energy generated by afuel cell may power a notebook computer or an electronics device carriedby military personnel. In one embodiment, the present invention provides‘small’ fuel cells that are configured to output less than 200 watts ofpower (net or total). Fuel cells of this size are commonly referred toas ‘micro fuel cells’ and are well suited for use with portableelectronics devices. In one embodiment, the fuel cell is configured togenerate from about 1 milliwatt to about 200 Watts. In anotherembodiment, the fuel cell generates from about 5 Watts to about 60Watts. The fuel cell system may be a stand-alone system, which is asingle package that produces power as long as it has access to a) oxygenand b) hydrogen or a hydrogen source such as a hydrocarbon fuel. Onespecific portable fuel cell package produces about 20 Watts or about 45Watts, depending on the number of cells in the stack.

The fluid delivery system includes a pressure source and a differentialflow meter. The pressure source may include a pump or a pressurizedbladder in a storage device. The differential flow meter uses a flowrestrictor and a sensor. The pressure source moves a fluid through theflow restrictor; the sensor detects differential pressure in the flowrestrictor and outputs a signal that permits dynamic control of fluidflow, e.g., by controlling a pump or valve. A differential pressuresensor measures the pressure drop across the flow restrictor, and thispressure drop can be used to determine flow rate across the restrictor.A linear or polynomial equation, for example, may be used to assess flowrate based on a measured differential pressure across the restrictor.The flow restrictor increases readability of the fluid flow, whichimproves feedback and control accuracy in a micro fuel cell system whereflow rates are small.

Fluids controlled in this manner may include any fluid inlet to a microfuel cell system. Suitable reactants include oxygen or air, hydrogen, ahydrogen source such as a hydrocarbon fuel, etc. Other examples aresuitable for use and several are provided below. As the term is usedherein, a ‘fluid’ may include a liquid (Newtonian or other), gas(including heated vapors for a liquid), or combinations thereof.

Fuel Cell System

FIG. 1A illustrates a fuel cell system 1 for producing electrical energyin accordance with one embodiment of the present invention. Fuel cellsystem 1 comprises a fuel cell 20 and a hydrogen storage device 14.

Hydrogen storage device 14 stores and outputs hydrogen, which may be apure source such as compressed hydrogen stored in a container. Hydrogenstorage device 14 may also include a solid-hydrogen storage system suchas a metal or carbon-based hydrogen storage device known to those ofskill in the art. An outlet of hydrogen storage device 14 detachablycouples to fuel delivery system 15 (or some intermediate line orplumbing) so that storage device 14 may be replaced when depleted.Hydrogen storage device 14 may be a single use device or reusable.

Fuel delivery system 11 transfers hydrogen from storage device 14 tofuel cell 20. Fuel delivery system 11 may also regulate oxygen provisionto fuel cell 20. Fuel delivery system 11 may include a pressure sourcesuch as a pump, one or more tubes (or ‘lines’) that communicate a fluid(liquid and/or gas), a differential flow meter, a sensor, one or morevalves such as a shutoff valve, and other plumbing components. Fueldelivery system 11 will be described in further detail below.

Fuel cell 20 electrochemically converts hydrogen and oxygen to water,generating electrical energy and heat in the process. Ambient airreadily supplies oxygen. A pure or direct oxygen source may also beused. The water often forms as a vapor, depending on the temperature offuel cell 20. For some fuel cells, the electrochemical reaction may alsoproduce carbon dioxide as a byproduct.

In one embodiment, fuel cell 20 is a low volume ion conductive membrane(PEM) fuel cell suitable for use with portable applications such asconsumer electronics. An ion conductive membrane fuel cell comprises amembrane electrode assembly that carries out the electrical energygenerating electrochemical reaction. The membrane electrode assemblyincludes a hydrogen catalyst, an oxygen catalyst, and an ion conductivemembrane that a) selectively conducts protons and b) electricallyisolates the hydrogen catalyst from the oxygen catalyst. A hydrogen gasdistribution layer contains the hydrogen catalyst and allows thediffusion of hydrogen therethrough. An oxygen gas distribution layercontains the oxygen catalyst and allows the diffusion of oxygen andhydrogen protons therethrough. The ion conductive membrane separates thehydrogen and oxygen gas distribution layers. In chemical terms, theanode comprises the hydrogen gas distribution layer and hydrogencatalyst, while the cathode comprises the oxygen gas distribution layerand oxygen catalyst.

In one embodiment, a PEM fuel cell includes a fuel cell stack having aset of bi-polar plates. A membrane electrode assembly is disposedbetween two bi-polar plates. Hydrogen distribution occurs via a channelfield on one plate while oxygen distribution occurs via a channel fieldon a second plate on the other side of the membrane electrode assembly.Specifically, a first channel field distributes hydrogen to the hydrogengas distribution layer, while a second channel field distributes oxygento the oxygen gas distribution layer. The term ‘bi-polar’ referselectrically to a bi-polar plate (whether comprised of one plate or twoplates) sandwiched between two membrane electrode assembly layers. Inthe stack, the bi-polar plate acts as both a negative terminal for oneadjacent (e.g., above) membrane electrode assembly and a positiveterminal for a second adjacent (e.g., below) membrane electrode assemblyarranged on the opposite face of the bi-polar plate.

In electrical terms, the anode includes the hydrogen gas distributionlayer, hydrogen catalyst and bi-polar plate. The anode acts as thenegative electrode for fuel cell 20 and conducts electrons that arefreed from hydrogen molecules so that they can be used externally, e.g.,to power an external circuit. In a fuel cell stack, the bi-polar platesare connected in series to add electrical potential gained in each layerof the stack. In electrical terms, the cathode includes the oxygen gasdistribution layer, oxygen catalyst and bi-polar plate. The cathoderepresents the positive electrode for fuel cell 20 and conducts theelectrons back from the external electrical circuit to the oxygencatalyst, where they can recombine with hydrogen ions and oxygen to formwater.

The hydrogen catalyst separates the hydrogen into protons and electrons.An ion conductive membrane blocks the electrons, and electricallyisolates the chemical anode (hydrogen gas distribution layer andhydrogen catalyst) from the chemical cathode. The ion conductivemembrane also selectively conducts positively charged ions.Electrically, the anode conducts electrons to a load (electrical energyis produced) or battery (energy is stored). Meanwhile, protons movethrough the ion conductive membrane. The protons and used electronssubsequently meet on the cathode side, and combine with oxygen to formwater. The oxygen catalyst in the oxygen gas distribution layerfacilitates this reaction. One common oxygen catalyst comprises platinumpowder very thinly coated onto a carbon paper or cloth. Many designsemploy a rough and porous catalyst to increase surface area of theplatinum exposed to the hydrogen and oxygen.

In one embodiment, fuel cell 20 comprises a set of bi-polar plates,where each bi-polar plate is formed from a single sheet of metal. Eachplate includes channel fields on opposite surfaces of the thin metalsheet. The single bi-polar plate thus dually distributes hydrogen andoxygen: one channel field distributes hydrogen while a channel field onthe opposite surface distributes oxygen. Multiple bi-polar plates can bestacked to produce a ‘fuel cell stack’ in which a membrane electrodeassembly is disposed between each pair of adjacent bi-polar plates. Inanother embodiment, each bi-polar plate is formed from multiple layersthat include more than one sheet of metal.

Since the electrical generation process in fuel cell 20 is exothermic,fuel cell 20 may implement a thermal management system to dissipateheat. Fuel cell 20 may also employ a number of humidification plates(HP) to manage moisture levels in the fuel cell.

While the present invention will mainly be discussed with respect to PEMfuel cells, it is understood that the present invention may be practicedwith other fuel cell architectures. The main difference between fuelcell architectures is the type of ion conductive membrane used. Inanother embodiment, fuel cell 20 is phosphoric acid fuel cell thatemploys liquid phosphoric acid for ion exchange. Solid oxide fuel cellsemploy a hard, non-porous ceramic compound for ion exchange and may besuitable for use with the present invention. Generally, any fuel cellarchitecture may be applicable to the fluid control designs describedherein. Other such fuel cell architectures include direct methanol,alkaline and molten carbonate fuel cells, for example.

A fuel cell system of the present invention may also use a ‘reformed’hydrogen supply. FIG. 1B illustrates a fuel cell system 10 for producingelectrical energy in accordance with another embodiment of the presentinvention. Fuel cell system 10 differs from system 1 in that it reformsa fuel source to provide hydrogen. Fuel cell system 10 comprises a fuelprocessor 15, fuel delivery system 11 and a fuel cell 20. Fuel cell 20was described above, and fuel delivery system 11 will be describedfurther below.

Fuel processor 15 processes a fuel source 17 to produce hydrogen. Fuelsource 17 acts as a carrier for hydrogen and can be manipulated toseparate hydrogen. Fuel source 17 may include any hydrogen bearing fuelstream, hydrocarbon fuel, or other hydrogen fuel source. Hydrocarbonfuel sources 17 suitable for use with the present invention includemethanol, ethanol, gasoline, propane, butane and natural gas, forexample. Other fuel sources may be used with a fuel cell package of thepresent invention, such as sodium borohydride. Several hydrocarbon andammonia products may also be used.

Fuel source 17 may be stored as a fuel mixture. When the fuel processor15 comprises a steam reformer, storage device 16 contains a fuel mixtureof a hydrocarbon fuel source and water. Hydrocarbon fuel source/waterfuel mixtures are frequently represented as a percentage fuel source inwater. In one embodiment, fuel source 17 comprises methanol or ethanolconcentrations in water in the range of 1%-99.9%. Other liquid fuelssuch as butane, propane, gasoline, military grade “JP8” etc. may also becontained in storage device 16 with concentrations in water from 5-100%.In a specific embodiment, fuel source 17 includes 67% methanol byvolume.

As shown, the reformed hydrogen supply comprises a fuel processor 15 anda fuel source storage device 16. Storage device 16 stores fuel source17, and may comprise a refillable and/or disposable (single use) fuelcartridge. Either cartridge offers a user instant recharging. Both typesof cartridges may also be either vented or non-vented. Vented cartridgesinclude a small hole, single direction flow valve, hydrophobic filter orother configuration to allow air to enter the fuel cartridge as liquidis displaced from the cartridge. Non-vented cartridges may rely on aninternal bladder disposed within a vented outer case. The bladder is asealed fuel container that prevents air and fuel from mixing as fuel isdisplaced, and the outer case provides mechanical strength to thecartridge. This type of cartridge allows for “orientation” independentoperation. Non-vented cartridges may also be pressurized, by eliminatingthe vent hole in the outer case, and substituting a pressure source suchas from a propellant like propane or compressed nitrogen gas forexample, or a pressurized process gas used on other fluid streams withinthe fuel cell system, such as the cathode air inlet gas stream forexample. Other suitable designs include other components such as wicksthat move a liquid fuel from locations within a fuel cartridge to acartridge exit.

Fuel processor 15 processes the hydrocarbon fuel source 17 and outputshydrogen. A hydrocarbon fuel processor 15 heats and processes ahydrocarbon fuel source 17 in the presence of a catalyst to producehydrogen. Fuel processor 15 comprises a reformer, which is a catalyticdevice that converts a liquid or gaseous hydrocarbon fuel source 17 intohydrogen and carbon dioxide. As the term is used herein, reformingrefers to the process of producing hydrogen from a fuel source. Onesuitable fuel processor 15 is described in further detail below.

In one embodiment, fuel processor 15 is a steam reformer that only needssteam and fuel to produce hydrogen. Several types of reformers suitablefor use in fuel cell system 10 include steam reformers, auto thermalreformers (ATR) or catalytic partial oxidizers (CPOX). ATR and CPOXreformers mix air with the fuel and steam mix. ATR and CPOX systemsreform fuels such as methanol, diesel, regular unleaded gasoline andother hydrocarbons. In a specific embodiment, storage device 16 providesmethanol 17 to fuel processor 15, which reforms the methanol at about280° C. or less and allows fuel cell system 10 use in low temperatureapplications.

A fuel cell 20 may be configured to receive hydrogen from either adirect hydrogen supply 12 or a reformed source. Fuel cell 20 typicallyreceives hydrogen from one supply at a time, although fuel cell packagesthat employ redundant hydrogen provision from multiple supplies areuseful in some applications.

Regardless of the fuel cell system design, the system requires reactantfluid provision. Fluid delivery system 11 thus provides one or morereactant fluids to the corresponding locations in a fuel cell system. Ascan be seen from the above two systems, there are several fluids thatfluid delivery system 11 can deliver, and several locations where thefluid delivery system 11 can be situated to do so.

FIG. 1C illustrates schematic operation for the fuel cell system 10 ofFIG. 1B in accordance with a specific embodiment of the presentinvention. As shown, system 10 includes a fluid delivery system 11 forfuel source provision, fuel processor 15, fuel cell 20 and an air pump41. A fuel container 16 couples to system 10 and stores fuel source 17.

Fuel container 16 stores methanol or a methanol mixture as a hydrogenfuel source 17. An outlet of fuel container 16 couples to quickdisconnect 23, which communicates methanol 17 into hydrogen fuel sourceline 25. In one embodiment, quick disconnect 23 is included in fluiddelivery system 11 and permits detachable coupling between a fuelcontainer 16 and a package that includes fuel cell 20 and fuel processor15. In this case, line 25 is internal to the package.

Fluid delivery system 11 regulates methanol supply into system 10.Within fluid delivery system 11, line 25 divides into two lines: a firstline 27 that transports methanol 17 to a heater (also referred to hereinas a ‘burner’) 30 for fuel processor 15 and a second line 29 thattransports methanol 17 to a reformer 32 in fuel processor 15. Lines 25,27 and 29 may comprise channels disposed in the fuel processor or tubesleading thereto, for example.

Flow control is provided on each line 27 and 29. Separate pumps 21 a and21 b are provided for lines 27 and 29, respectively, to pressurize eachline separately and transfer methanol at independent rates, if desired.A model 030SP-S6112 pump as provided by Biochem, NJ is suitable totransmit liquid methanol on either line in a specific embodiment. A flowrestriction 24 a and 24 b and sensor 28 a and 28 b are also provided oneach line 27 and 29, respectively. Each sensor 28, situated betweenstorage device 16 and fuel processor 18, detects pressure for methanol17 flow through its corresponding restriction 24 as the methanoltransfers between storage device 16 and fuel processor 18. The sensor 28then outputs a signal indicative of detected pressure. In conjunctionwith suitable control, such as digital control applied by a processorthat implements instructions from stored software, each pump 21 respondsto control signals from the processor and moves a desired amount ofmethanol 17 from storage device 16 to reformer 32 on each line 27 and29. The interaction of pump 21, flow restriction 24 and a sensor aredescribed in further detail below with respect to FIGS. 2A-C. In oneembodiment, the control system knows how much fuel is being pumped, andthe control system communicates this information to a chip on storagedevice 16.

Air pump 41 delivers oxygen and air from the ambient room through line31 to the cathode in the fuel cell 20, where some oxygen is used in thecathode to generate electricity. Air pump 41 may include a fan, bloweror compressor, for example. High operating temperatures in fuel cell 20also heat the oxygen and air. In the embodiment shown, the heated oxygenand air is then transmitted via line 33 to regenerator 36 of fuelprocessor 15, where it is additionally heated before entering heater 30.This double pre-heating increases efficiency of the fuel cell system bya) reducing heat lost to reactants in heater 30 (such as fresh oxygenthat would otherwise be near room temperature), b) cooling the fuel cellduring energy production. In this embodiment, a model BTC compressor asprovided by Hargraves, NC is suitable to pressurize oxygen and air forfuel cell system 10.

A fan 37 blows cooling air (e.g. from the ambient room) over fuel cell20. Fan 37 may be suitable sized to move air as desired by heatingrequirements of the fuel cell; and many vendors known to those of skillin the art provide fans suitable for use with package 10.

Fuel processor 15 receives methanol 17 and outputs hydrogen. Fuelprocessor 15 comprises heater 30, reformer 32, boiler 34 and regenerator36. Heater (or burner) 30 includes an inlet (which may be extended toinclude a boiler in some cases) that receives methanol 17 from line 27and a catalyst that generates heat with methanol presence. Boiler 34includes an inlet that receives methanol 17 from line 29. The structureof boiler 34 permits heat produced in heater 30 to heat methanol 17 inboiler 34 before reformer 32 receives the methanol 17. Boiler 34includes an outlet that provides heated methanol 17 to reformer 32.Reformer 32 includes an inlet that receives heated methanol 17 fromboiler 34. A catalyst in reformer 32 reacts with the methanol 17 andproduces hydrogen and carbon dioxide. This reaction is slightlyendothermic and draws heat from heater 30. A hydrogen outlet of reformer32 outputs hydrogen to line 39. In one embodiment, fuel processor 15also includes a preferential oxidizer that intercepts reformer 32hydrogen exhaust and decreases the amount of carbon monoxide in theexhaust. The preferential oxidizer employs oxygen from an air inlet tothe preferential oxidizer and a catalyst, such as ruthenium or platinum,that is preferential to carbon monoxide over hydrogen.

Regenerator 36 pre-heats air before the air enters heater 30.Regenerator 36 also reduces heat loss from package 10 by heating airbefore the heat escapes fuel processor 15. In one sense, regeneratoruses waste heat in fuel processor 15 to increase thermal management andthermal efficiency of the fuel processor. Specifically, waste heat fromheater 30 pre-heats incoming air provided to heater 30 to reduce heattransfer to the air in the heater so more heat transfers to reformer 32.The regenerator also functions as insulation for the fuel processor, byreducing the overall amount of heat loss of the fuel processor.

Line 39 transports hydrogen from fuel processor 15 to fuel cell 20. In aspecific embodiment, gaseous delivery lines 31, 33 and 39 includechannels in a metal interconnect that couples to both fuel processor 15and fuel cell 20. A hydrogen flow sensor (not shown) may also be addedon line 39 to detect and communicate the amount of hydrogen beingdelivered to fuel cell 20. In conjunction with the hydrogen flow sensorand suitable control, such as digital control applied by a processorthat implements instructions from stored software, fuel processor 15regulates hydrogen gas provision to fuel cell 20.

Fuel cell 20 includes a hydrogen inlet port that receives hydrogen fromline 39 and includes a hydrogen intake manifold that delivers the gas toone or more bi-polar plates and their hydrogen distribution channels. Anoxygen inlet port of fuel cell 20 receives oxygen from line 31; anoxygen intake manifold receives the oxygen from the port and deliversthe oxygen to one or more bi-polar plates and their oxygen distributionchannels. An anode exhaust manifold 38 collects gases from the hydrogendistribution channels and delivers them to the ambient room, or back tothe fuel processor. A cathode exhaust manifold collects gases from theoxygen distribution channels and delivers them to a cathode exhaust portand line 33, or to the ambient room.

In addition to the components shown in shown in FIG. 1C, system 10 mayalso include other elements such as electronic controls, additionalpumps and valves, added system sensors, manifolds, heat exchangers andelectrical interconnects useful for carrying out functionality of a fuelcell system 10 that are known to one of skill in the art and omittedherein for sake of brevity.

FIG. 1C shows one specific plumbing arrangement for a fuel cell system;other plumbing arrangements are suitable for use herein. In a specificembodiment, line 29 runs inlet methanol 17 across a heat exchanger thatreceives heat from the exhaust of the heater 30 in fuel processor 15.This increases thermal efficiency for the system by preheating theincoming fuel source (to reduce heating in the burner 30) andrecuperates heat that would otherwise be expended from the system. Inanother specific embodiment, the exhaust of the heater 30 in fuelprocessor 15 is carried to one or more heat transfer appendages in fuelcell 20 during system start-up to expedite reaching initial elevatedoperating temperatures in the fuel cell 20. An additional fan may alsobe used to blow air over one or more heat transfer appendages thatpermit conductive heat transfer with internal portions of a fuel cellstack. This provides dedicated cooling of the stack.

Fluid Delivery System

FIG. 2A shows a fluid delivery system 11 a in accordance with oneembodiment of the present invention. Fluid delivery system 11 a is wellsuited for accurately delivering a methanol fuel mixture from acartridge or other portable storage device to a micro fuel cell system.Although the present invention will now primarily be discussed withrespect to movement of liquid methanol, it is understood that fluidcontrol systems described herein are well suited for use with gases andother liquids used in a fuel cell system, such as any fuel source orreactant (hydrogen, oxygen, etc.) in a fuel cell or fuel processor.Fluid delivery system 11 a includes an inlet line 25, pump 21, adifferential flow meter 22 that includes flow restriction 24 and sensor28, a controller 200, and outlet line 27.

Pump 21 pressurizes methanol 17 and moves it from inlet line 25 todifferential flow meter 22, before use of methanol 17 in one or moredesired downstream components via outlet line 27. For system 10 of FIG.1C, pump 21 b moves methanol 17 from storage device 16 to the reformer32 in fuel processor 15 and through any plumbing between the source anddestination. Similarly, pump 21 a moves fuel source 17 from storagedevice 16 to a heater in processor 15 and through any plumbingtherebetween. Pump 21 may include any suitable design, such as adiaphragm, screw type, electro-static, field-induced, peristaltic,piezo-actuated, piston, MEMS, roots, electrostatic, electrohydrodynamicrotary vane, centrifugal, solenoid, syringe or gear pump, for example.Other pump types are suitable for use herein.

The present invention is well suited for use with low volume fluid pumps(“micro pumps”). Flow rates for a micro pump are typically small and mayvary based on the fuel cell system and whether the fluid being pumped isa liquid or gas.

A liquid fuel source 17 is common when the fuel source is stored in aportable storage device and pumped to a fuel processor for production ofhydrogen. In one embodiment, pump 21 is a micro-pump configured to movea liquid fuel source 17 at a flow rate that is less than about 4milliliters per hour per watt output by the fuel cell. In a specificembodiment, the fuel source is liquid methanol (which includes methanolmixtures) and pump 21 is configured to move less than about 1 milliliterper hour per watt. Thus, one fuel cell system that includes a 45 wattfuel cell may use a micro pump that moves about 45 milliliters per hourof methanol to a fuel processor, while a 200 watt fuel cell may use amicro pump that moves about 200 milliliters per hour of methanol. Theseflow rates may vary with size, design, and efficiency of the fuel cellsystem and its components; other liquid flow rates are suitable for useherein. For example, flow rates may be larger or smaller than thoseprovided above based on the hydrocarbon fuel source/water mixture ratio,efficiency of the fuel processor and fuel cell, ambient temperature,ambient pressure, number of start/stop cycles on the system, totalnumber of operating hours on the system, efficiency of the powerregulation circuits etc.

Volumetric flow rates are typically higher for gases than liquids. Twocommon reactant gases used in a fuel cell system are oxygen andhydrogen. In one embodiment when fluid delivery system 11 a moves airfor oxygen supply, pump 21 is configured to move the air at a flow ratethat is less than about 0.25 liters per minute per watt output by thefuel cell. In a specific embodiment, an airflow rate of less than about0.12 liters per minute per watt is suitable. For example, a 10-watt fuelcell may draw about 1.2 liters per minute of air and oxygen. Otheroxygen flow rates may be used and will vary, for example, if the air isused for other purposes such as cooling, providing additional oxygen todown stream components such as a catalytic heater located in the samefluidic path downstream of the fuel cell cathode, or supplyingadditional air to a separate reactor such as a preferential oxidizer(PROX) located upstream of the fuel cell in a separate fluidic path(e.g., the fuel side or hydrogen path of the fuel cell). Additionally,flow rates may be larger or smaller than those provided above based onthe hydrocarbon fuel source/water mixture ratio, efficiency of the fuelprocessor and fuel cell, ambient temperature, ambient pressure, numberof start/stop cycles on the system, total number of operating hours onthe system, efficiency of the power regulation circuits etc. For examplea 45 W fuel cell system operating at 10,000 ft altitude may require upto 8 liters per minute of air during steady state conditions. Thevolumetric air flow may also be sized to accommodate startup of thesystem such as in the case of a reformed methanol fuel cell system wherethe air flow is used to heat up and cool down the fuel processor; insome instances, the fuel cell stack may not be producing any powerduring heat up and cool down. For example a 10 W reformed methanol fuelcell system may require 6 liters per minute during startup conditions.

Accurate control of hydrogen flow rates is common for a direct hydrogensupply system (see FIG. 1A for example). In one embodiment when fluiddelivery system 11 a moves hydrogen gas, pump 21 is configured to movethe hydrogen gas at a flow rate that is less than about 60 millilitersper minute per watt output by the fuel cell. In a specific embodiment, ahydrogen flow rate of less than about 18 milliliters per minute per wattis suitable. For example, a 60-watt fuel cell may draw about 1.2 litersper minute of hydrogen. Other hydrogen flow rates may be used and willvary, for example, based on fuel cell efficiency, type of fuel cell,membrane electrode assembly materials and catalysts, ambienttemperature, ambient pressure, number of start/stop cycles on thesystem, total number of operating hours on the system, efficiency of thepower regulation circuits etc.

Fluid delivery system 11 a also includes a differential flow meter 22that includes a flow restriction 24 and at least one sensor 28 thatmeasures differential pressure across the differential flow meter. Theflow restriction 24 alters fluid flow through its structure and improvesdetection of the low flow rate using a sensor.

In one embodiment, flow restriction 24 includes an orifice with asmaller opening (e.g., diameter) than a line (such as line 25 or anotherintermediate tube or line) that supplies the fuel source to the flowrestriction. The smaller diameter increases flow rate in the flowrestriction and improves detection of the low flow rate. In a specificembodiment, the orifice has a diameter that is less than about 0.002inches. Other orifice sizes may be used and will vary for example, onviscosity of the fluid and whether the fluid is a liquid or a gas.Examples of different orifice diameters, pressure drops and flow ratesare shown in the tables 1 and 2.

TABLE 1 67% Desired Orifice Methanol Pressure Tube Diameter Flow Drop IDRatio Inlet ID/ (in) ml/hr [psig] [inch] Orifice ID 0.0088 200 0.5 0.0323.61696005 0.0048 60 0.5 0.064 13.20727072 0.0034 30 0.5 0.0329.338950691 0.0024 15 0.5 0.064 26.41454145 0.0063 200 1 0.0325.115153957 0.0034 60 1 0.064 18.67790138 0.0024 30 1 0.032 13.207270720.0017 15 1 0.064 37.35580276 0.0028 200 5 0.032 11.43783196 0.0015 60 50.064 41.76505717 0.0011 30 5 0.032 29.53235514 0.0008 15 5 0.06483.53011433

TABLE 2 P2-P1 Orifice Dia 67% methanol flow [psig] [inch] [ml/hr] 4.740.0017 70 4.40 0.0017 65 4.06 0.0017 60 3.72 0.0017 55 3.39 0.0017 503.05 0.0017 45 2.71 0.0017 40 2.37 0.0017 35 2.03 0.0017 30 1.69 0.001725 1.35 0.0017 20 1.02 0.0017 15 0.68 0.0017 10

Flow restriction 24 is configured to develop a pressure differentialthat is neither too high for the pressure source nor too low forpressure sensor 24. For example, if a solenoid pump is used to supplymethanol 17, some models of solenoid pumps may have a maximum pumpingpressure of about 4 psig. Hence the pressure drop caused by the flowrestriction should be less than 4 psig at full flow rate, so that thepump has enough pressure to overcome a) the pressure drop of the fuelcell fluidic path and b) the differential pressure caused by flowrestriction 24. If the differential pressure is too low, say less than0.1 psig for some sensors for example, then the resultant differentialpressure reading produced by the pressure sensor may be too low for thecontrol circuit to accurately and repeatedly record. Pressure drops lessthan 0.1 psig can be used, depending on the sensor employed.

Flow meter 22 may also affect total power consumption in the fuel cellsystem. If sensor 28 is applied to an air stream produced by an aircompressor, then the resulting differential pressure caused by the flowrestriction could increase the power required to drive the compressor ata specified airflow rate. To avoid such parasitic performance andimprove net power for the fuel cell system, flow restriction 24 may bedesigned and configured as an integral part of the system to avoidparasitic pressure losses. For example, the cathode flow path has arelatively constant flow resistance, about 0.5 psig for example, andsensor 28 may be configured to measure the differential pressure acrossthe cathode inlet and exit; as a result, no additional flow restrictionis added by sensor 28 that increases parasitic power consumption.

Pressure may also be used to convey flow rates and performance suitablefor use herein. In one embodiment, for liquid fuels fed by a pump,differential pressure at maximum rated flow is about 0.5 psig to about 5psig. For liquid fuels fed by a pressurized fuel cartridge, adifferential pressure may be as high as about 10 psig to about 20 psigat maximum rated flow. For hydrogen fuel, a differential pressure atmaximum rated flow between about 2 psig and about 5 psig is suitable foruse. For reactant air streams, a differential pressure at maximum ratedflow about 0.5 psig is suitable if the air stream is supplied by acompressor, and about 0.05 inches-water pressure if the air stream issupplied by a fan or blower. The desired pressure drops listed above mayvary depending on the type of pressure source, available sensortechnology, etc. Sizing of the pressure drop caused by the pressurerestriction is dependant on many aspects of a complete fuel cell system,pressurized or non-pressurized liquid fuels, pressure rating of the pumpor compressor, pressure rating of available pressure sensingtechnologies etc. Other differential pressures may be used for each ofthe reactants in a fuel cell system.

In another embodiment, differential flow meter 22 includes a tube thatis adapted to improve detection and sensing of a parameter used in fluidflow rate assessment. In a specific embodiment, the tube is lengthenedto decrease flow rate sensitivity to upstream and downstreamdisturbances and to provide a more stable flow and flow rate along thelength of the tube. The tube may be coiled to increase its length whileminimizing space in a small volume fuel cell package. In a specificembodiment, the tube is greater than about 2 centimeters in length. Tubelengths greater than about 4 centimeters in length are also suitable foruse.

At least one sensor 28 measures differential pressure of acrosslocations of differential flow meter 22, and outputs a signal includinginformation indicative of the sensed pressure(s) to controller 200.Fluid delivery system 11 a of FIG. 2A includes one sensor; system 11 bof FIG. 2B includes two sensors. For FIG. 2B, the two sensors 28 detectfluid pressure at two locations of the flow restriction 24, each providea signal to controller 200, which then compares the measured values todetermine a pressure difference. Sensing may be done directly in thefluid or indirectly, e.g., on an outside surface of the tube. The twolocations may include any two portions of flow restriction 24 ordifferential flow meter 22, such as relatively opposite ends of a tube.Alternatively, the two locations may include inlet and outlet ends of anarrowing orifice. In one embodiment, the fuel cell system packageincludes a circuit board that supports controller 200, and sensor 28attaches to the circuit board. This decreases size of the fuel cellpackage and permits simple mounting and assembly of components of fluiddelivery system 11 a. In a specific embodiment, sensor 28 is a model 24PC as provided by Honeywell Corp. of Freeport, Ill.

Controller 200 receives a signal from sensor 28 and is configured toconvert information in the signal to a command that affects flow rate ofmethanol 17. Controller 200 is described in further detail below withrespect to FIG. 2C.

FIG. 2B shows a fluid delivery system 11 b in accordance with anotherembodiment of the present invention. Fluid delivery system 11 b includesan inlet valve 23, inlet line 25, diaphragm pump 21 d, a differentialflow meter 22 that includes flow a coiled flow restriction 24 d and twosensors 28, controller 200, and outlet line 27.

Diaphragm pump 21 d moves liquid methanol 17. A diaphragm pump (alsoreferred to as a ‘membrane pump’) is a small pump that includes achamber bordered by a diaphragm whose displacement is controllable.Applying a control signal to the diaphragm pump 21 d excites an actuatorthat causes the diaphragm to translate back and forth, perpendicular toa diaphragm face. This displaces volume in the chamber and moves thefluid. Two check valves, oriented to permit flow in opposite directionsrelative to the chamber, allow fuel to enter and exit the chamber.Actuating the diaphragm hence moves the methanol 17 from the intake toexhaust check valve in response to a control signal by controller 200.

Diaphragm pumps 21 d are inexpensive and compact pumps, available from awide variety of vendors, and offer reduced power consumption.Piezoelectric actuation in a diaphragm pump is well suited for pump 21.The exact pump used will depend on whether the pump moves a liquid or agas, a desired flow rate, and size demands. One suitable diaphragm pumpfor moving a liquid in a fuel cell system is a model LPD-100 availablefrom PAR Technologies of Hampton, Va. A suitable diaphragm pump formoving a gas (e.g., air or hydrogen) in a fuel cell system is a modelRTP32A03 available from Okenseiko of Japan. The specifications for thesepumps are available from their respective manufactures and areincorporated herein by reference in their entirety and for all purposes;other diaphragm pumps are suitable for use herein. Many of thesecommercially available pumps are configured to consume less than about 2watts when moving a fluid, which is of value to portable fuel cellsystems since power consumed by the pump affects net power efficiency ofthe portable system.

Despite size and power advantages of diaphragm pumps 21 d, these pumpshave not been used in micro-fuel cell systems due to their inherent lackof controllability. Diaphragm pump actuators permit control of:displacement of the diaphragm and/or amplitude of the diaphragmdisplacement. Since a diaphragm pump 21 d has a large diameter relativeto the tubes that feed it and that it pumps into, small fluctuations inthe downstream flow resistance lead to large changes in flow rate. In anideal system, the downstream flow resistance is constant, and hence itwould be possible to accurately gauge pumped flow rate if the pumpfrequency and or amplitude are known.

Realistically however, downstream flow resistance of micro fuel cellsystems constantly changes. Many factors such as manufacturingtolerances, temperature variations, and/or vaporizer pressureoscillations (e.g., resulting from a boiler blow-down) cause thedownstream flow resistance to vary and become unstable. When the flowresistance becomes unstable, it is typically not possible to calculatefluid flow rate based solely on the actuation frequency/amplitude of adiaphragm pump.

In order to overcome these control problems with a diaphragm pump oranother low flow rate pump, the fluid delivery system 11 b adds adifferential flow meter 22 to provide accurate flow rate assessment andcontrol of fluids moved by diaphragm pump 21 d, regardless of thedownstream components and pressure disturbances. In this case,differential flow meter 22 includes two sensors 28 located at oppositeends of a coiled tube flow restriction 24. FIGS. 4A and 4B, as describedbelow, illustrate the difference in control by adding differential flowmeter 22. In one embodiment, the fluid delivery system 11 b componentsare integrated into a single unit. In another embodiment, the fluiddelivery system 11 b components are discrete devices.

An additional issue associated with commercially available diaphragmpumps is that they generally do not provide shutoff capability when notin use, and cannot be used to throttle fluid flow from high to lowpressure. Therefore, if a methanol storage device acquires a higherpressure than the downstream system (e.g., the storage device is left ona dashboard of a car for some time and heats up), then pump 21 may leakfuel into the downstream catalytic systems. This leakage can permanentlydamage the fuel cell system, or lead to unwanted release of fluids intothe environment outside the fuel cell system.

To overcome such upstream pressure and leakage issues, fluid deliverysystem 11 includes an inlet valve 23 that provides shutoff capability.The shutoff valve permits fluidic disconnect between an upstream linethat communicates with the storage device and a line that communicatesthe methanol to a fuel processor or another downstream component. Thefluidic disconnect prevents fuel from leaking through pump 21 whenpressure in the fuel supply or fuel storage device rises above thedownstream system pressure. This may occur when the fuel cell system isoff and stored in a hot location, for example. In a specific embodiment,shutoff valve 23 is included in a quick disconnect fitting, at theinterface of the storage device and a package housing that contains thefuel cell system; the quick disconnect allows for rapid replacement ofthe storage device. Shutoff valve 23 thus solves the problem ofunexpected high pressure in an upstream source sending fluids to anunprepared fuel processor and fuel cell system.

Controller 200 sends control signals to valve 23 and permits automatedon/off control of methanol 17 into system 11 a. Thus, controller 200 maydisconnect shutoff valve 23 when a fuel cell system is turned off, notproducing hydrogen in the fuel processor, and/or not generatingelectricity.

Although the present invention has been described so far with respect toa pump used as a pressure source to move fluid, other pressure sourcesmay be employed in fluid delivery system 11. This may include anymechanism that moves a fluid, including those that do not provide activecontrol, such as a single speed compressor or single speed pump. FIG. 2Cshows a fluid delivery system 1 ic that uses a pressurized storagedevice 16 in accordance with another embodiment of the presentinvention. Fluid delivery system 11 c includes storage device 16, inletline 25, flow control valve 23 c, a differential flow meter 22,controller 200, and outlet line 27.

Pressurized storage device 16 is configured to provide a relativelysteady pressure that does not allow for feedback-based control. Storagedevice 16 may include a foam or propellant that expands and pushes on acollapsible bladder (e.g., rubber) in the storage device, whichmaintains an elevated pressure in the bladder so as to move fluid fromthe bladder into line 25. Other mechanisms may be used in storage device16 to move the fuel source therefrom, such as another process fluid thatapplies a steady pressure to the bladder.

For a less controllable pressure source such as this, the presentinvention uses valve 23 to regulate flow rate of the methanol 17. Inthis case, fluid delivery system 11 c employs feedback from a flowsensor 28 to alter opening (e.g., magnitude, rate, etc.) of the valve toproduce a desired flow rate for the fluid. Valve 23 may include aproportional type valve with variable opening characteristics, or it maybe a nominally open or closed valve. A valve with shutoff capability mayalso be used. Valve 23 may also permit flow rate control in real time.

Controller 200 is configured to convert a signal output by the at leastone sensor 28 to a command that affects flow rate of the fuel source.The controlled device may vary with the fuel cell system, as one ofskill in the art will appreciate. For the schematic shown in FIG. 1C,controller 200 may output commands to: pumps 21 a and 21 b, valve 23,air compressor 41 for inlet to the cathode, a blower 37 for cooling thefuel cell 20, and a blower that passes air over one or more heattransfer appendages in the fuel cell for cooling of the fuel cell stack,if used. If package 10 is electrical load following, then controller 200meters fuel source 17 to deliver fuel source 17 at a flow ratedetermined by a desired power level output of fuel cell 20. For fuelprovision to the heater, fluid delivery system 11 may deliver fuelsource 17 to the heater to maintain a certain temperature in the heateror inlet fuel source. In one embodiment, a separate controller 200 isused for fluid delivery system 11. In another embodiment, control offluid delivery system 11 is integrated into a main fuel cell controlsystem.

As shown in FIG. 2C, controller 200 includes a processor 202 and amemory 204. Processor 202 may include a commercially availablemicroprocessor such as one of the Intel or Motorola family of chips, areduced instruction set computer (RISC) chip such as the PowerPC™microprocessor available from Motorola, Inc, or any other suitableprocessor. Memory 204 may comprise some form of digital storage such asa mass storage or RAM available to processor 202 and configured to storecontrol programs and data. Regardless of controller 200 configuration,it may employ one or more memories or memory modules configured to storeprogram instructions for controlling fuel cell and fluid delivery system11 systems described herein. Such memory or memories may also beconfigured to store data structures, control programs, or other specificnon-program information described herein.

Many of the methods and techniques described herein constitute systemcontrols and will comprise digital control applied by control logic thatimplements instructions from stored software. The stored instructionsmay correspond to any methods or elements described herein. The controllogic includes any combination of hardware and software needed forcontrol. For example, the control logic may include instructions storedin memory 204 that are executed by processor 202. Input/output logic maybe employed to facilitate communication between processor 202 andcomponents of fluid delivery system 11.

A differential pressure measures the pressure drop across the flowrestrictor, and this pressure drop can be used to determine flow rateacross the restrictor. A linear or polynomial equation, for example, maybe used to assess flow rate based on a measured differential pressureacross the restrictor. The equation may be stored in memory 204 ascomputer instructions or a program run by processor 202 that convertspressure-related signals received from sensor 28 to flow rate controlcommands on pump 21 or valve 23.

Fuel delivery system 11 may also accommodate for temperature. As shownin FIG. 2C, fuel delivery system 11 c includes a temperature sensor 210that measures temperature of methanol 17 flowing in line 21, or atanother location within the micro fuel cell system, such as the methanolflowing through a pump 21. Temperature sensor 210 may also detect theambient temperature. Using a reading of the fuel temperature, acalibration curve for fluid control can be adjusted to account formaterial property changes of the fluid (such as viscosity) that areaffected by temperature.

FIG. 3 shows a method 300 for controlling fluid delivery in a fuel cellsystem in accordance with one embodiment of the present invention. Asmentioned above, the fluid may include any reactant for the fuel cellsystem such as hydrogen, oxygen or a fuel source converted to hydrogenby a fuel processor.

Process flow 300 may start when a system controller receives a call forelectrical energy production in a fuel cell, or a call for fuel sourceflow. To begin, the controller may open a disconnect or shutoff valve,if one is included in the fuel cell system to prevent flow.

The controller then sends a command to begin movement of the fluid at apredetermined flow rate (302). Actuation of a diaphragm pump maycommence at a preset frequency or amplitude. In this case, the fluidtravels to the pump, which pressurizes the fluid and pumps it to one ormore downstream components. Alternatively, a controller may send asignal to a proportional valve, appropriate for the desired flow rate,if the valve is used to regulate fluid flow from a steady pressuresource.

The fluid then flows through the flow restriction (304) and a propertyof the fluid flow is detected while the fluid is in the flow restriction(306). In one embodiment, differential pressure is measured in multipleportions of the flow restriction.

The measured differential pressure is then used to determine flow rateof the fluid (308). A pre-determined formula stored in software permitsautomated conversion and control in real time. This assumes some form ofcalibrated relationship between pressure (delta p) and flow in the flowrestriction (see FIG. 4A for example). Flow rate estimation may alsoaccommodate for temperature. In this case, a sensor detects temperatureof the fluid or ambient temperature, and the controller uses themeasured temperature to adjust a calibration curve, when appropriate(e.g., to the nearest temperature curve for the fluid).

Process flow 300 then changes flow rate of the fluid, if needed (310).For example, if the flow is incorrect by more than +1-5%, diaphragmfrequency and/or amplitude of a diaphragm pump may be changed. Steps304-310 may repeat until the flow is within 5% of a desired flow rate.Different error bounds and correction schemes may be used, such as 1-2%,or less. In this manner, the controller and fluid delivery systemprovide an adaptive system that achieves a desired flow rate based onmeasured performance. As described above, the desired flow rate willvary with the fuel cell system and fluid being moved. In one specificfuel cell system, a 45 W reformed methanol micro fuel cell system pumpsabout 45-60 milliliters per hour of methanol; the same system may alsomove 2-8 liters per minute of air.

If the system controller receives call to halt fuel flow, then actuationof the diaphragm ceases and/or the shutoff valve is closed.

Data plotted in a chart shown in FIG. 4A illustrates the controllabilityimparted by fluid delivery system 11 b and process flow 300. The testsused to derive the data were performed using a diaphragm pump 21, avariable metering valve to serve as flow restrictor 24, and two pressuresensors 28 that provided differential pressure reading across the flowrestrictor 24. Data in the chart shows that the flow rate passingthrough flow restrictor 24 can be accurately measured, independent ofthe actual pumping mechanism.

By contrast, FIG. 4B shows flow data in a flow vs. actuation frequencyof a diaphragm pump 21 for “restriction 1” of FIG. 4A. As can be seen,the data in FIG. 4B does not offer a reliable method for relating fluidflow to diaphragm pump actuation frequency (which is analogous torevolutions per minute in a centrifugal pump). Comparing the charts inFIGS. 4A and 4B, it becomes clear that the present invention permitsreliable controllability for micro fuel cells and other applications.

Fuel Cell System Package

The present invention is well suited for use in a reduced-size andportable fuel cell package. A fuel package refers to a fuel cell systemthat receives hydrogen, or a hydrogen fuel source, and outputselectrical energy. At a minimum, this includes a fuel cell. The packageneed not include a cover or housing, e.g., in the case where a fuelcell, or a fuel cell and fuel processor, is included in a battery bay ofa laptop computer. In this case, the fuel cell package only includes thefuel cell, or fuel cell and fuel processor, and no housing. The packagemay include a compact profile, low volume, or low mass—any of which isuseful in any power application where size is relevant. As the term isused herein, fuel cell package and fuel cell system are synonymous,where package is used to more conveniently express volume and powerdensity.

In one embodiment, the fuel cell package includes a fuel cell, a fuelprocessor, and dedicated connectivity between the two. The dedicatedconnectivity may provide a) fluid or gas communication between the fuelprocessor and the fuel cell, and/or b) structural support between thetwo or for the package. In one embodiment, a dedicated interconnectprovides much of the connectivity. Assembling the fuel processor andfuel cell together in a common and substantially enclosed packageprovides a portable ‘black box’ package that receives a hydrogen fuelsource and outputs electrical energy.

FIG. 5A illustrates a fuel cell package 400 in accordance with oneembodiment of the present invention. Package 400 provides compact andportable electrical energy generation using fuel cell technology.

An outer housing 402 contains a fuel cell. Housing 402 providesmechanical protection for internal components within its boundaries, andmay include any shape or configuration to provide such protection.Housing 402 includes a number of openings for air intake and exhaust.Opening 404 allows air from the ambient room or environment to enterpackage 400, e.g., to cool a fuel cell contained therein or for energygeneration in the fuel cell. Opening 406 acts as an exhaust port forheated gases after they acquire heat from the fuel cell, which typicallyoperates at an elevated temperature relative to air in the ambientenvironment. While openings 404 and 406 are shown as somewhat linearslits, the openings may comprise any dimensions suitable for intake andexhaust of cooling air (or oxygen used in a fuel processor). Inaddition, the package may include less or greater than two openings.

Volume may characterize package 400. The volume includes all componentsof the package used in the system to generate electricity, save astorage device used to supply hydrogen or a fuel source. In oneembodiment, the volume includes the fuel cell and any componentsexternal to housing 402 used to generate electricity (e.g., not justcomponents included within housing 402, such as a pump used for fueldelivery disposed partially outside the housing), and/or a powerconditioner that converts the fuel cell output voltage to a levelrequired by a power consumer and which may be turned on or off by thefuel cell control system as needed. In one embodiment, package 400 has atotal volume less than about a liter. In a specific embodiment, package400 has a total volume less than about ½ liter. Greater and lesserpackage volumes may be used with the present invention.

Package 400 also includes a relatively small mass. In one embodiment,package 400 has a total mass less than about a 1 kg. In a specificembodiment, package 400 has a total volume less than about ½ liter.Greater and lesser package masses are possible.

FIG. 5B shows a perspective view of a coplanar fuel cell system in asingle package 420 in accordance with one embodiment of the presentinvention. Package 420 includes fuel cell 20 and fuel processor 15,arranged adjacent to each other.

Fuel cell 20 is shown with a housing 422 that includes top plate 64 anda number of sidewalls 424. Sidewall 424 a includes two openings: acooling air intake 428 and an exhaust 430. Cooling fan 37 of FIG. 1C isdisposed relatively close and internal to intake 428 or exhaust 430.

For package 420, fuel pumps 21 are included for fluid delivery andattached to an external housing of the package. Fuel pumps 21 may employa solenoid pump, syringe pump or any other commercially available pumpthat moves a fuel. FIG. 5B also shows an air intake pipe 432 (line 31 ofFIG. 1C) that communicates oxygen and air from the ambient room orenvironment, through the package housing, and to fuel cell 20 for use inthe cathode.

FIG. 5C illustrates a perspective view of internal components for a fuelcell package 440 in accordance with another embodiment of the presentinvention.

Package 440 includes a block chassis 442 that acts as a structuralframework to which functional components of package 440 are attached. Inone embodiment, chassis 442 forms a bottom wall of an external housingfor package 440. Chassis 442 includes a suitably stiff material, such asa metal or rigid plastic. Aluminum, Fr₄, carbon fiber, ABS and steel areall suitable for use. Alternatively, any material that providesmechanical integrity and includes a low thermal conductance may be used.

Package 440 also includes fluid lines and connections 444 incorporatedinto fuel cell 20 and fuel processor 15, as opposed to separate tubesand hoses between the fuel cell 20 and processor 15. This decreases sizefor package 440. Pump 21 provides fuel source movement and is coupled toa bracket that attaches to chassis 442. An air compressor 448 providesair to the fuel cell cathode and is attached to chassis 442. An intakeplenum 445 is included to guide air between an outer housing of package440 and inlet port 428 of fuel cell 20.

Control board 452 includes suitable software and hardware forcontrolling components within package 440. Hardware may include acommercially available processor, such as any of those available in theIntel, MicroChip or Motorola family of processors. Some form of memoryis also included. Random-access memory (RAM) and read-only memory (ROM)may be included to store fuel delivery program instructions, implementedby the processor, that execute control functions for one more componentsof a fuel cell system. The control board may also include a device toallow for reprogramming of the control system firmware without the needto remove the control board.

The control system may also include one or more batteries, capacitors orother energy storage devices wired in a serial and/or parallel, andwired in parallel with the fuel cell which serve to allow for hybridizedpower output. The control circuit may include other devices to allow theenergy storage devices to share the net load on the system with the fuelcells, or to take up the whole load of the system if the fuel cell isnot producing power, or to be completed removed from the load output.Additional circuitry may be provided for the fuel cells to recharge theenergy storage devices in the case that they are rechargeable batteriesor capacitors, or any other energy storage device that can beelectrically recharged.

An electrical adapter may also be included in the package (not shown inFIG. 5C, and can also be part of control board 452) converts electricalenergy output by fuel cell 20 to a suitable level as determined bydesign of package 440. For example, package 440 may be used as atethered adapter to power a laptop computer, in which electrical adapter446 converts electrical energy output by fuel cell 20 to a voltage andcurrent suitable for electrical provision to the laptop. DC/DCconversion is typical, but other power conditioning may also be applied.The electrical adaptor, or power regulator, may also have the capabilityto be turned on or off as needed, and may include load levelingcapabilities such as provided by capacitors on the input and outputlines. Adapter 446 may also include a hardware interface that receives awire that couples to the electronics device.

Package 440 may also includes additional fuel cell system componentssuch as a cathode air inlet 31, a fuel feed from a detachable fuelsource cartridge that couples to package 440, and a sensor and wires fortemperature sensing.

Although fuel cell packages have been largely described with respect tofuel processor inclusion, a package of the present invention need notinclude a processor. In another embodiment, the package only includes afuel cell that receives hydrogen from a supply coupled to the package.The package then provides a portable black box that receives hydrogenand outputs electrical energy. Since the volume has decreased, thisprovides fuel cell packages with less volume and mass—for the same poweroutput—and thus even greater power densities.

In one embodiment, the present invention provides a tethered fuel cellpackage. A tethered package refers to a fuel cell package including atether that allows electrical coupling to the package from a distance,and typically includes a conductor capable of communicating electricalenergy from a fuel cell or electrical adapter included in the package toan electronics device. In one embodiment, the tether includes a wiredetachably coupled to the package and configured to transmit DCelectricity generated by the fuel cell. Since the fuel cell package isportable, tethering the package provides a portable form of electricalpower that may be plugged into one or multiple portable electronicsdevices.

Fuel Cell

FIG. 6A illustrates a cross sectional view of a fuel cell stack 60 foruse in fuel cell 20 in accordance with one embodiment of the presentinvention. FIG. 6B illustrates an outer top perspective view of a fuelcell stack 60 and fuel cell 20 in accordance with another embodiment ofthe present invention.

Referring initially to FIG. 6A, fuel cell stack 60 includes a set ofbi-polar plates 44 and a set of membrane electrode assembly (MEA) layers62. Two MEA layers 62 neighbor each bi-polar plate 44. With theexception of topmost and bottommost membrane electrode assembly layers62 a and 62 b, each MEA 62 is disposed between two adjacent bi-polarplates 44. For MEAs 62 a and 62 b, top and bottom end plates 64 a and 64b include a channel field 72 on the face neighboring an MEA 62.

The bi-polar plates 44 in stack 60 also each include one or more heattransfer appendages 46. As shown, each bi-polar plate 44 includes a heattransfer appendage 46 a on one side of the plate and a heat transferappendage 46 b on the opposite side. Heat transfer appendages 46 arediscussed in further detail below.

As shown in FIG. 6A, stack 60 includes twelve membrane electrodeassembly layers 62, eleven bi-polar plates 44 and two end plates 64(FIG. 6B shows 18 plates 44 in the stack). The number of bi-polar plates44 and MEA layers 62 in each set may vary with design of fuel cell stack60. Stacking parallel layers in fuel cell stack 60 permits efficient useof space and increased power density for fuel cell 20 and a fuel cellpackage 10 including fuel cell 20. In one embodiment, each membraneelectrode assembly 62 produces 0.7 V and the number of MEA layers 62 isselected to achieve a desired voltage. Alternatively, the number of MEAlayers 62 and bi-polar plates 44 may be determined by the allowablethickness of package 10. A fuel cell stack 60 having from one MEA 62 toseveral hundred MEAs 62 is suitable for many applications. A stack 60having from about three MEAs 62 to about twenty MEAs 62 is also suitablefor numerous applications. Fuel cell 20 size and layout may also betailored and configured to output a given power.

Referring to FIG. 6B, top and bottom end plates 64 a and 64 b providemechanical protection for stack 60. End plates 64 also hold the bi-polarplates 44 and MEA layers 62 together, and apply pressure across theplanar area of each bi-polar plate 44 and each MEA 62. End plates 64 mayinclude steel or another suitably stiff material. Bolts 82 a-d connectand secure top and bottom end plates 64 a and 64 b together.

Fuel cell 20 includes two anode manifolds (84 and 86). Each manifolddelivers a product or reactant gas to or from the fuel cell stack 60.More specifically, each manifold delivers a gas between a verticalmanifold created by stacking bi-polar plates 44 (FIG. 6D) and plumbingexternal to fuel cell 20. Inlet hydrogen manifold 84 is disposed on topend plate 64 a, couples with an inlet line to receive hydrogen gas, andopens to an inlet hydrogen manifold 102 (FIG. 6D) that is configured todeliver inlet hydrogen gas to a channel field 72 on each bi-polar plate44 in stack 60. Outlet manifold 86 receives outlet gases from an anodeexhaust manifold 104 (FIG. 6D) that is configured to collect wasteproducts from the anode channel fields 72 of each bi-polar plate 44.Outlet manifold 86 may provide the exhaust gases to the ambient spaceabout the fuel cell. In another embodiment, manifold 86 provides theanode exhaust to line 38, which transports the unused hydrogen back tothe fuel processor during start-up.

Fuel cell 20 includes two cathode manifolds: an inlet cathode manifoldor inlet oxygen manifold 88, and an outlet cathode manifold or outletwater/vapor manifold 90. Inlet oxygen manifold 88 is disposed on top endplate 64 a, couples with an inlet line (line 31, which draws air fromthe ambient room) to receive ambient air, and opens to an oxygenmanifold 106 (FIG. 6D) that is configured to deliver inlet oxygen andambient air to a channel field 72 on each bi-polar plate 44 in stack 60.Outlet water/vapor manifold 90 receives outlet gases from a cathodeexhaust manifold 108 (FIG. 6D) that is configured to collect water(typically as a vapor) from the cathode channel fields 72 on eachbi-polar plate 44.

As shown in FIG. 6B, manifolds 84, 86, 88 and 90 include molded channelsthat each travel along a top surface of end plate 64 a from theirinterface from outside the fuel cell to a manifold in the stack. Eachmanifold or channel acts as a gaseous communication line for fuel cell20 and may comprise a molded channel in plate 64 or a housing of fuelcell 20. Other arrangements to communicate gases to and from stack 60are contemplated, such as those that do not share common manifolding ina single plate or structure.

FIG. 6C illustrates an ion conductive membrane fuel cell (PEMFC)architecture 120 for use in fuel cell 20 in accordance with oneembodiment of the present invention. As shown, PEMFC architecture 120comprises two bi-polar plates 44 and a membrane electrode assembly layer(or MEA) 62 sandwiched between the two bi-polar plates 44. The MEA 62electrochemically converts hydrogen and oxygen to water and generateselectrical energy and heat in the process. Membrane electrode assembly62 includes an anode gas diffusion layer 122, a cathode gas diffusionlayer 124, a hydrogen catalyst 126, ion conductive membrane 128, anodeelectrode 130, cathode electrode 132, and oxygen catalyst 134.

Pressurized hydrogen gas (H₂) enters fuel cell 20 via hydrogen port 84,proceeds through inlet hydrogen manifold 102 and through hydrogenchannels 74 of a hydrogen channel field 72 a disposed on the anode face75 of bi-polar plate 44 a. The hydrogen channels 74 open to anode gasdiffusion layer 122, which is disposed between the anode face 75 ofbi-polar plate 44 a and ion conductive membrane 128. The pressure forceshydrogen gas into the hydrogen-permeable anode gas diffusion layer 122and across the hydrogen catalyst 126, which is disposed in the anode gasdiffusion layer 122. When an H₂ molecule contacts hydrogen catalyst 126,it splits into two H+ ions (protons) and two electrons (e−). The protonsmove through the ion conductive membrane 128 to combine with oxygen incathode gas diffusion layer 124. The electrons conduct through the anodeelectrode 130, where they build potential for use in an external circuit(e.g., a power supply of a laptop computer) After external use, theelectrons flow to the cathode electrode 132 of PEMFC architecture 120.

Hydrogen catalyst 126 breaks hydrogen into protons and electrons.Suitable catalysts 126 include platinum, ruthenium, and platinum blackor platinum carbon, and/or platinum on carbon nanotubes, for example.Anode gas diffusion layer 122 comprises any material that allows thediffusion of hydrogen therethrough and is capable of holding thehydrogen catalyst 126 to allow interaction between the catalyst andhydrogen molecules. One such suitable layer comprises a woven ornon-woven carbon paper. Other suitable gas diffusion layer 122 materialsmay comprise a silicon carbide matrix and a mixture of a woven ornon-woven carbon paper and Teflon.

On the cathode side of PEMFC architecture 120, pressurized air carryingoxygen gas (O₂) enters fuel cell 20 via oxygen port 88, proceeds throughinlet oxygen manifold 106, and through oxygen channels 76 of an oxygenchannel field 72 b disposed on the cathode face 77 of bi-polar plate 44b. The oxygen channels 76 open to cathode gas diffusion layer 124, whichis disposed between the cathode face 77 of bi-polar plate 44 b and ionconductive membrane 128. The pressure forces oxygen into cathode gasdiffusion layer 124 and across the oxygen catalyst 134 disposed in thecathode gas diffusion layer 124. When an O₂ molecule contacts oxygencatalyst 134, it splits into two oxygen atoms. Two H+ ions that havetraveled through the ion selective ion conductive membrane 128 and anoxygen atom combine with two electrons returning from the externalcircuit to form a water molecule (H₂O). Cathode channels 76 exhaust thewater, which usually forms as a vapor. This reaction in a single MEAlayer 62 produces about 0.7 volts.

Cathode gas diffusion layer 124 comprises a material that permitsdiffusion of oxygen and hydrogen protons therethrough and is capable ofholding the oxygen catalyst 134 to allow interaction between thecatalyst 134 with oxygen and hydrogen. Suitable gas diffusion layers 124may comprise carbon paper or cloth, for example. Other suitable gasdiffusion layer 124 materials may comprise a silicon carbide matrix anda mixture of a woven or non-woven carbon paper and Teflon. Oxygencatalyst 134 facilitates the reaction of oxygen and hydrogen to formwater. One common catalyst 134 comprises platinum. Many designs employ arough and porous catalyst 134 to increase surface area of catalyst 134exposed to the hydrogen or oxygen. For example, the platinum may resideas a powder very thinly coated onto a carbon paper or cloth cathode gasdiffusion layer 124.

Ion conductive membrane 128 electrically isolates the anode from thecathode by blocking electrons from passing through membrane 128. Thus,membrane 128 prevents the passage of electrons between gas diffusionlayer 122 and gas diffusion layer 124. Ion conductive membrane 128 alsoselectively conducts positively charged ions, e.g., hydrogen protonsfrom gas diffusion layer 122 to gas diffusion layer 124. For fuel cell20, protons move through membrane 128 and electrons are conducted awayto an electrical load or battery. In one embodiment, ion conductivemembrane 128 comprises an electrolyte. One electrolyte suitable for usewith fuel cell 20 is Celtec 1000 from PEMEAS USA AG of Murray Hill, N.J.(www.pemeas.com). Fuel cells 20 including this electrolyte are generallymore carbon monoxide tolerant and may not require humidification. Ionconductive membrane 128 may also employ a phosphoric acid matrix thatincludes a porous separator impregnated with phosphoric acid.Alternative ion conductive membranes 128 suitable for use with fuel cell20 are widely available from companies such as United technologies,DuPont, 3M, and other manufacturers known to those of skill in the art.For example, WL Gore Associates of Elkton, Md. produces the primeaSeries 58, which is a low temperature MEA suitable for use with thepresent invention.

In one embodiment, fuel cell 20 requires no external humidifier or heatexchanger and the stack 60 only needs hydrogen and air to produceelectrical power. Alternatively, fuel cell 20 may employ humidificationof the cathode to fuel cell 20 improve performance. For some fuel cellstack 60 designs, humidifying the cathode increases the power andoperating life of fuel cell 20.

FIG. 6D illustrates a top perspective view of a stack of bi-polar plates(with the top two plates labeled 44 p and 44 q) in accordance with oneembodiment of the present invention. Bi-polar plate 44 is a single plate44 with first channel fields 72 disposed on opposite faces 75 of theplate 44.

Functionally, bi-polar plate 44 a) delivers and distributes reactantgases to the gas diffusion layers 122 and 124 and their respectivecatalysts, b) maintains separation of the reactant gasses from oneanother between MEA layers 62 in stack 60, c) exhausts electrochemicalreaction byproducts from MEA layers 62, d) facilitates heat transfer toand/or from MEA layers 62 and fuel cell stack 60, and e) includes gasintake and gas exhaust manifolds for gas delivery to other bi-polarplates 44 in the fuel stack 60.

Structurally, bi-polar plate 44 has a relatively flat profile andincludes opposing top and bottom faces 75 a and 75 b (only top face 75 ais shown) and a number of sides 78. Faces 75 are substantially planarwith the exception of channels 76 formed as troughs into substrate 89.Sides 78 comprise portions of bi-polar plate 44 proximate to edges ofbi-polar plate 44 between the two faces 75. As shown, bi-polar plate 44is roughly quadrilateral with features for the intake manifolds, exhaustmanifolds and heat transfer appendage 46 that provide outer deviationfrom a quadrilateral shape.

The manifold on each plate 44 is configured to deliver a gas to achannel field on a face of the plate 44 or receive a gas from thechannel field 72. The manifolds for bi-polar plate 44 include aperturesor holes in substrate 89 that, when combined with manifolds of otherplates 44 in a stack 60, form an inter-plate 44 gaseous communicationmanifold (such as 102, 104, 106 and 108). Thus, when plates 44 arestacked and their manifolds substantially align, the manifolds permitgaseous delivery to and from each plate 44.

Bi-polar plate 44 includes a channel field 72 or “flow field” on eachface of plate 44. Each channel field 72 includes one or more channels 76formed into the substrate 89 of plate 44 such that the channel restsbelow the surface of plate 44. Each channel field 72 distributes one ormore reactant gasses to an active area for the fuel cell stack 60.Bi-polar plate 44 includes a first channel field 72 a on the anode face75 a of bi-polar plate 44 that distributes hydrogen to an anode (FIG.6C), while a second channel field on opposite cathode face 75 bdistributes oxygen to a cathode. Specifically, channel field 72 aincludes multiple channels 76 that permit oxygen and air flow to anodegas diffusion layer 122, while channel field 72 b includes multiplechannels 76 that permit oxygen and air flow to cathode gas diffusionlayer 124. For fuel cell stack 60, each channel field 72 is configuredto receive a reactant gas from an intake manifold 102 or 106 andconfigured to distribute the reactant gas to a gas diffusion layer 122or 124. Each channel field 72 also collects reaction byproducts forexhaust from fuel cell 20. When bi-polar plates 44 are stacked togetherin fuel cell 60, adjacent plates 44 sandwich an MEA layer 62 such thatthe anode face 75 a from one bi-polar plate 44 neighbors a cathode face75 b of an adjacent bi-polar plate 44 on an opposite side of the MEAlayer 62.

Bi-polar plate 44 may include one or more heat transfer appendages 46.Each heat transfer appendage 46 permits external thermal management ofinternal portions of fuel cell stack 60. More specifically, appendage 46may be used to heat or cool internal portions of fuel cell stack 60 suchas internal portions of each attached bi-polar plate 44 and anyneighboring MEA layers 62, for example. Heat transfer appendage 46 islaterally arranged outside channel field 72. In one embodiment,appendage 46 is disposed on an external portion of bi-polar plate 44.External portions of bi-polar plate 44 include any portions of plate 44proximate to a side or edge of the substrate included in plate 44.External portions of bi-polar plate 44 typically do not include achannel field 72. For the embodiment shown, heat transfer appendage 46substantially spans a side of plate 44 that does not include intake andoutput manifolds 102-108. For the embodiment shown in FIG. 6A, plate 44includes two heat transfer appendages 46 that substantially span bothsides of plate 44 that do not include a gas manifold. Peripherallydisposing heat transfer appendage 46 allows heat transfer between innerportions of plate 44 and the externally disposed appendage 46 via theplate substrate 89.

Heat may travel to or form the heat transfer appendage 46. In otherwords, appendage 46 may be employed as a heat sink or source. Thus, heattransfer appendage 46 may be used as a heat sink to cool internalportions of bi-polar plate 44 or an MEA 62. Fuel cell 20 employs acooling medium to remove heat from appendage 46. Alternatively, heattransfer appendage 46 may be employed as a heat source to provide heatto internal portions of bi-polar plate 44 or an MEA 62. In this case, acatalyst may be disposed on appendage 46 to generate heat in response tothe presence of a heating medium.

For cooling, heat transfer appendage 46 permits integral conductive heattransfer from inner portions of plate 44 to the externally disposedappendage 46. During hydrogen consumption and electrical energyproduction, the electrochemical reaction generates heat in each MEA 62.Since internal portions of bi-polar plate 44 are in contact with the MEA62, a heat transfer appendage 46 on a bi-polar plate 44 thus cools anMEA 62 adjacent to the plate via a) conductive heat transfer from MEA 62to bi-polar plate 44 and b) lateral thermal communication and conductiveheat transfer from central portions of the bi-polar plate 44 in contactwith the MEA 62 to the external portions of plate 44 that includeappendage 46. In this case, heat transfer appendage 46 sinks heat fromsubstrate 89 between a first channel field 72 on one face 75 of plate 44and a second channel field 72 on the opposite face of plate 44 to heattransfer appendage 46 in a direction parallel to a face 75 of plate 44.When a fuel cell stack 60 includes multiple MEA layers 62, lateralthermal communication through each bi-polar plate 44 in this mannerprovides interlayer cooling of multiple MEA layers 62 in stack60—including those layers in central portions of stack 60.

Fuel cell 20 may employ a cooling medium that passes over heat transferappendage 46. The cooling medium receives heat from appendage 46 andremoves the heat from fuel cell 20. Heat generated internal to stack 60thus conducts through bi-polar plate 44, to appendage 46, and heats thecooling medium via convective heat transfer between the appendage 46 andcooling medium. Air is suitable for use as the cooling medium.

Heat transfer appendage 46 may be configured with a thickness that isless than the thickness between opposite faces 75 of plate 44. Thereduced thickness of appendages 46 on adjacent bi-polar plates 44 in thefuel cell stack 60 forms a channel between adjacent appendages. Multipleadjacent bi-polar plates 44 and appendages 46 in stack form numerouschannels. Each channel permits a cooling medium or heating medium topass therethrough and across heat transfer appendages 46. In oneembodiment, fuel cell stack 60 includes a mechanical housing thatencloses and protects stack 60. Walls of the housing also provideadditional ducting for the cooling or heating medium by forming ductsbetween adjacent appendages 46 and the walls.

The cooling medium may be a gas or liquid. Heat transfer advantagesgained by high conductance bi-polar plates 44 allow air to be used as acooling medium to cool heat transfer appendages 46 and stack 60. Forexample, a dc-fan 37 may be attached to an external surface of themechanical housing. The fan 37 moves air through a hole in themechanical housing, through the channels between appendages to cool heattransfer appendages 46 and fuel cell stack 60, and out an exhaust holeor port in the mechanical housing. Fuel cell system 10 may then includeactive thermal controls based on temperature sensed feedback. Increasingor decreasing coolant fan speed regulates the amount of heat removalfrom stack 60 and the operating temperature for stack 60. In oneembodiment of an air-cooled stack 60, the coolant fan speed increases ordecreases as a function of the actual cathode exit temperature, relativeto a desired temperature set-point.

For heating, heat transfer appendage 46 allows integral heat transferfrom the externally disposed appendage 46 to inner portions of plate 44and any components and portions of fuel cell 20 in thermal communicationwith inner portions of plate 44. A heating medium passed over the heattransfer appendage 46 provides heat to the appendage. Heat convectedonto the appendage 46 then conducts through the substrate 89 and intointernal portions of plate 44 and stack 60, such as portions of MEA 62and its constituent components.

In one embodiment, the heating medium comprises a heated gas having atemperature greater than that of appendage 46. Exhaust gases from heater30 or reformer 32 of fuel processor 15 may each include elevatedtemperatures that are suitable for heating one or more appendages 46.

In another embodiment, fuel cell comprises a catalyst 192 (FIG. 6A)disposed in contact with, or in proximity to, one or more heat transferappendages 46. The catalyst 192 generates heat when the heating mediumpasses over it. The heating medium in this case may comprise any gas orfluid that reacts with catalyst 192 to generate heat. Typically,catalyst 192 and the heating medium employ an exothermic chemicalreaction to generate the heat. Heat transfer appendage 46 and plate 44then transfer heat into the fuel cell stack 60, e.g. to heat internalMEA layers 62. For example, catalyst 192 may comprise platinum and theheating medium includes the hydrocarbon fuel source 17. The fuel source17 may be heated to a gaseous state before it enters fuel cell 20. Thisallows gaseous transportation of the heating medium and gaseousinteraction between the fuel source 17 and catalyst 192 to generateheat. Similar to the cooling medium described above, a fan disposed onone of the walls then moves the gaseous heating medium within fuel cell20.

In a specific embodiment, the hydrocarbon fuel source 17 used to reactwith catalyst 192 comes from a reformer exhaust (see FIG. 1C, line 35)or heater exhaust in fuel processor 15. This advantageously pre-heatsthe fuel source 17 before receipt within fuel cell 20 and alsoefficiently uses or burns any fuel remaining in the reformer or heaterexhaust after processing by fuel processor 15. Alternatively, fuel cell20 may include a separate hydrocarbon fuel source 17 feed that directlysupplies hydrocarbon fuel source 17 to fuel cell 20 for heating andreaction with catalyst 192. In this case, catalyst 192 may compriseplatinum. Other suitable catalysts 192 include palladium, aplatinum/palladium mix, iron, ruthenium, and combinations thereof. Eachof these will react with a hydrocarbon fuel source 17 to generate heat.Other suitable heating mediums include hydrogen or any heated gasesemitted from fuel processor 15, for example.

When hydrogen is used as the heating medium, catalyst 192 comprises amaterial that generates heat in the presence of hydrogen, such aspalladium or platinum. As will be described in further detail below, thehydrogen may include hydrogen supplied from the reformer 32 in fuelprocessor 15 as exhaust.

As shown in FIG. 6A, catalyst 192 is arranged on, and in contact with,each heat transfer appendage 46. In this case, the heating medium passesover each appendage 46 and reacts with catalyst 192. This generatesheat, which is absorbed via conductive thermal communication by thecooler appendage 46. Wash coating may be employed to dispose catalyst192 on each appendage 46. A ceramic support may also be used to bondcatalyst 192 on an appendage 46.

For catalyst-based heating, heat then a) transfers from catalyst 192 toappendage 46, b) moves laterally though bi-polar plate 44 via conductiveheat transfer from lateral portions of the plate that include heattransfer appendage 46 to central portions of bi-polar plate 44 incontact with the MEA layers 62, and c) conducts from bi-polar plate 44to MEA layer 62. When a fuel cell stack 60 includes multiple MEA layers62, lateral heating through each bi-polar plate 44 provides interlayerheating of multiple MEA layers 62 in stack 60, which expedites fuel cell20 warm up.

Bi-polar plates 44 of FIG. 6A include heat transfer appendages 46 oneach side. In this case, one set of heat transfer appendages 46 a isused for cooling while the other set of heat transfer appendages 46 b isused for heating. Bi-polar plates 44 illustrated in FIG. 6D show plates44 with four heat transfer appendages 46 disposed on three sides ofstack 60. Appendage 46 arrangements can be otherwise varied to affectand improve heat dissipation and thermal management of fuel cell stack60 according to other specific designs. For example, appendages 46 neednot span a side of plate 44 as shown and may be tailored based on howthe heating fluid is channeled through the housing.

Although the present invention provides a bi-polar plate 44 havingchannel fields 72 that distribute hydrogen and oxygen on opposing sidesof a single plate 44, many embodiments described herein are suitable foruse with conventional bi-polar plate assemblies that employ two separateplates for distribution of hydrogen and oxygen.

While the present invention has mainly been discussed so far withrespect to a reformed methanol fuel cell (RMFC), the present inventionmay also apply to other types of fuel cells, such as a solid oxide fuelcell (SOFC), a phosphoric acid fuel cell (PAFC), a direct methanol fuelcell (DMFC), or a direct ethanol fuel cell (DEFC). In this case, fuelcell 20 includes components specific to these architectures, as one ofskill in the art will appreciate. A DMFC or DEFC receives and processesa fuel. More specifically, a DMFC or DEFC receives liquid methanol orethanol, respectively, channels the fuel into the fuel cell stack 60 andprocesses the liquid fuel to separate hydrogen for electrical energygeneration. For a DMFC, channel fields 72 in the bi-polar plates 44distribute liquid methanol instead of hydrogen. Hydrogen catalyst 126described above would then comprise a suitable anode catalyst forseparating hydrogen from methanol. Oxygen catalyst 128 would comprise asuitable cathode catalyst for processing oxygen or another suitableoxidant used in the DMFC, such as peroxide. In general, hydrogencatalyst 126 is also commonly referred to as an anode catalyst in otherfuel cell architectures and may comprise any suitable catalyst thatremoves hydrogen for electrical energy generation in a fuel cell, suchas directly from the fuel as in a DMFC. In general, oxygen catalyst 128may include any catalyst that processes an oxidant in used in fuel cell20. The oxidant may include any liquid or gas that oxidizes the fuel andis not limited to oxygen gas as described above. An SOFC, PAFC or MCFCmay also benefit from inventions described herein, for example. In thiscase, fuel cell 20 comprises an anode catalyst 126, cathode catalyst128, anode fuel and oxidant according to a specific SOFC, PAFC or MCFCdesign.

Fuel Processor

FIG. 7A illustrates a perspective view of components included in a fuelprocessor 15 in accordance with one embodiment of the present invention.FIG. 3B illustrates a cross-sectional front view of monolithic structure100. Fuel processor 15 reforms methanol to produce hydrogen. Fuelprocessor 15 comprises monolithic structure 100, end plates 182 and 184,end plate 185, reformer 32, heater 30, boiler 34, boiler 108, dewar 150and housing 152. Although the present invention will now be describedwith respect to methanol consumption for hydrogen production, it isunderstood that fuel processors of the present invention may consumeanother fuel source.

As the term is used herein, ‘monolithic’ refers to a single andintegrated structure that includes at least portions multiple componentsused in fuel processor 15. As shown in FIG. 3B, monolithic structure 100includes reformer 32, burner 30, boiler 34 and boiler 108. Monolithicstructure 100 also includes associated plumbing inlets and outlets forreformer 32, burner 30 and boiler 34 disposed on end plates 182 and 184and interconnect 200. Monolithic structure 100 comprises a commonmaterial 141 that constitutes the structure. The monolithic structure100 and common material 141 simplify manufacture of fuel processor 15.For example, using a metal for common material 141 allows monolithicstructure 100 to be formed by extrusion. In a specific embodiment,monolithic structure 100 is consistent in cross sectional dimensionsbetween end plates 182 and 184 and solely comprises copper formed in asingle extrusion.

Referring to FIG. 7B, housing 152 provides mechanical protection forinternal components of fuel processor 15 such as burner 30 and reformer32. Housing 152 also provides separation from the environment externalto processor 15 and includes inlet and outlet ports for gaseous andliquid communication in and out of fuel processor 15. Housing 152includes a set of housing walls that at least partially contain a dewar150 and provide external mechanical protection for components in fuelprocessor 15. The walls may comprises a suitably stiff material such asa metal or a rigid polymer, for example. Dewar 150 improves thermal heatmanagement for fuel processor 15 by a) allowing incoming air to bepre-heated before entering burner 30, b) dissipating heat generated byburner 32 into the incoming air before the heat reaches the outside ofhousing 152.

Boiler 34 heats methanol before reformer 32 receives the methanol.Boiler 34 receives methanol via a fuel source inlet on interconnect 200,which couples to a methanol supply line 27 (FIG. 1C). Since methanolreforming and hydrogen production via a catalyst 102 in reformer 32often requires elevated methanol temperatures, fuel processor 15pre-heats the methanol before receipt by reformer 32 via boiler 34.Boiler 34 is disposed in proximity to burner 30 to receive heatgenerated in burner 30. The heat transfers via conduction throughmonolithic structure from burner 30 to boiler 34 and via convection fromboiler 34 walls to the methanol passing therethrough. In one embodiment,boiler 34 is configured to vaporize liquid methanol. Boiler 34 thenpasses the gaseous methanol to reformer 32 for gaseous interaction withcatalyst 102.

Reformer 32 is configured to receive methanol from boiler 34. Walls 111in monolithic structure 100 and end walls 113 on end plates 182 and 184define dimensions for a reformer chamber 103. In one embodiment, endplate 182 and/or end plate 184 includes a channel that routes heatedmethanol exhausted from boiler 34 into reformer 32.

In one embodiment, a reformer includes a multi-pass arrangement.Reformer 32 includes three multi-pass portions that process methanol inseries: chamber section 32 a, chamber section 32 b, and chamber section32 c. A reformer chamber 103 then includes the volume of all threesections 32 a-c. Each section traverses the length of monolithicstructure 100; and opens to each other in series such that sections 32a-c form one continuous path for gaseous flow. More specifically, heatedand gaseous methanol from boiler 34 a) enters reformer chamber section32 a at an inlet end of monolithic structure 100 and flows to the otherend over catalyst 102 in section 32 a, b) then flows into chambersection 32 b at the second end of monolithic structure 100 and flows tothe inlet end over catalyst 102 in section 32 b, and c) flows intochamber section 32 c at one end of monolithic structure 100 and flows tothe other end over catalyst 102 in the chamber section 32 c.

Reformer 32 includes a catalyst 102 that facilitates the production ofhydrogen. Catalyst 102 reacts with methanol and produces hydrogen gasand carbon dioxide. In one embodiment, catalyst 102 comprises pelletspacked to form a porous bed or otherwise suitably filled into the volumeof reformer chamber 103. Pellet diameters ranging from about 50 micronsto about 1.5 millimeters are suitable for many applications. Pelletdiameters ranging from about 500 microns to about 1 millimeter aresuitable for use with reformer 32. Pellet sizes may be varied relativeto the cross sectional size of reformer sections 32 a-c, e.g., as thereformer sections increase in size so does catalyst 102 pelletdiameters. Pellet sizes and packing may also be varied to control thepressure drop that occurs through reformer chamber 103. In oneembodiment, pressure drops from about 0.2 to about 2 psi gauge aresuitable between the inlet and outlet of reformer chamber 103. Onesuitable catalyst 102 may include CuZn coated onto alumina pellets whenmethanol is used as a hydrocarbon fuel source 17. Other materialssuitable for catalyst 102 include platinum, palladium, aplatinum/palladium mix, nickel, and other precious metal catalysts forexample. Catalyst 102 pellets are commercially available from a numberof vendors known to those of skill in the art. Catalyst 102 may alsocomprise catalyst materials listed above coated onto a metal sponge ormetal foam. A wash coat of the desired metal catalyst material onto thewalls of reformer chamber 103 may also be used for reformer 32.

Reformer 32 is configured to output hydrogen and includes an outlet port209 that communicates hydrogen formed in reformer 32 outside of fuelprocessor 15. Port 209 is disposed on a wall of end plate 184 andincludes a hole that passes through the wall. Port 209 opens to hydrogenline in interconnect 200, which then forms part of a hydrogen provisionline 39. Line 39 communicates the hydrogen to the anode of fuel cell 20for electrical energy generation.

Hydrogen production in reformer 32 is slightly endothermic and drawsheat from burner 30. Burner 30 generates heat and is configured toprovide heat to reformer 32. As shown in FIG. 3B, burner 30 comprisesfour burner chambers 105 a-d that surround reformer 32. In oneembodiment, burner 30 uses electrical resistance and electrical energyto produce heat.

In the embodiment shown, burner 30 employs catalytic combustion toproduce heat. As the term is used herein, a burner refers to a heaterthat uses a catalytic heating process to generate heat. A heater in afuel processor of the present invention may alternatively employelectrical heating, for example. A catalyst 104 disposed in each burnerchamber 105 helps a burner fuel passed through the chamber generateheat. Burner 30 includes an inlet that receives methanol 17 from boiler108 via a channel in one of end plates 182 or 184. In one embodiment,methanol produces heat in burner 30 and catalyst 104 facilitates themethanol production of heat. In another embodiment, waste hydrogen fromfuel cell 20 produces heat in the presence of catalyst 104. Suitableburner catalysts 104 may include platinum or palladium coated ontoalumina pellets for example. Other materials suitable for catalyst 104include iron, tin oxide, other noble-metal catalysts, reducible oxides,and mixtures thereof. Catalyst 104 is commercially available from anumber of vendors known to those of skill in the art as small pellets.The pellets that may be packed into burner chamber 105 to form a porousbed or otherwise suitably filled into the burner chamber volume.Catalyst 104 pellet sizes may be varied relative to the cross sectionalsize of burner chamber 105. Catalyst 104 may also comprise catalystmaterials listed above coated onto a metal sponge or metal foam or washcoated onto the walls of burner chamber 105.

Some fuel sources generate additional heat in burner 30, or generateheat more efficiently, with elevated temperatures. Fuel processor 15includes a boiler 108 that heats methanol before burner 30 receives thefuel source. In this case, boiler 108 receives the methanol via fuelsource inlet 85. Boiler 108 is disposed in proximity to burner 30 toreceive heat generated in burner 30. The heat transfers via conductionthrough monolithic structure from burner 30 to boiler 108 and viaconvection from boiler 108 walls to the methanol passing therethrough.

Air including oxygen enters fuel processor 15 via air inlet port 91.Burner 30 uses the oxygen for catalytic combustion of methanol. A burner30 in fuel processor 15 generates heat and typically operates at anelevated temperature. In one embodiment, fuel processor 15 comprises adewar 150 to improve thermal management for fuel processor 15. Dewar 150at least partially thermally isolates components internal to housing152—such as burner 30—and contains heat within fuel processor 15. Dewar150 is configured such that air passing through dewar chamber 156receives heat generated in burner 30. Dewar 150 offers thus twofunctions for fuel processor 15: a) it permits active cooling ofcomponents within fuel processor 15 before the heat reaches an outerportion of the fuel processor, and b) it pre-heats the air going toburner 30. Air first passes along the outside of dewar 150 beforepassing through apertures in the dewar and along the inside of dewar150. This heats the air before receipt by air inlet port 93 of burner30.

In one embodiment, the fuel cell system runs anode exhaust from the fuelcell 20 back to fuel processor. As shown in FIG. 1C, line 38 routesunused hydrogen from fuel cell 20 burner inlet 109, which provides theanode exhaust to burner 30 (or to the regenerator 36 and then to burnerinlet 109 and into burner 30). Burner 30 includes a thermal catalystthat reacts with the unused hydrogen to produce heat. Since hydrogenconsumption within fuel cell 20 is often incomplete and the anodeexhaust often includes unused hydrogen, re-routing the anode exhaust toburner 30 allows the fuel cell system to capitalize on unused hydrogenin fuel cell 20 and increase hydrogen usage and efficiency. The fuelcell system thus provides flexibility to use different fuels in acatalytic burner 30. For example, if fuel cell 20 can reliably andefficiently consume over 90% of the hydrogen in the anode stream, thenthere may not be sufficient hydrogen to maintain reformer and boileroperating temperatures in fuel processor 15. Under this circumstance,methanol supply is increased to produce additional heat to maintain thereformer and boiler temperatures.

Burner inlet 109 traverses monolithic structure 100 and carries anodeexhaust from fuel cell 20 before provision into burner 30. Disposingburner inlet 109 adjacent to a burner chamber 105 also heats theincoming anode exhaust, which reduces heat transferred to the anodeexhaust in the burner chamber 105.

In another embodiment, the fuel cell system runs a heating medium fromfuel processor 15 to fuel cell 20 to provide heat to fuel cell 20. Inthis case, the fuel cell system includes plumbing configured totransport the heating medium from fuel processor 15 to fuel cell 20. Asthe term is used herein, plumbing may comprise any tubing, piping and/orchanneling that communicates a gas or liquid from one location to asecond location. The plumbing may also comprise one or more valves,gates or other devices to facilitate and control flow.

In a specific embodiment, line 35 transports heated gases to fan 37,which moves the heated gases within fuel cell 20 and across the fuelcell stack and heat transfer appendages (FIG. 1C). Alternatively, theplumbing may be configured to transport the heating medium from burner30 to one or more heat transfer appendages. In this case, line 35 maycontinue through the fuel cell housing and open in the proximity of oneor more heat transfer appendages. A hole in the fuel cell housing thenallows line 35 to pass therethrough or connect to a port thatcommunicates the gases to plumbing inside the fuel cell for delivery tothe fuel cell stack and heat transfer appendage. For catalytic heatgeneration in fuel cell 20, the plumbing may also transport the heatingmedium to facilitate gaseous interaction with the catalyst, such asplumbing delivery to one or more bulkheads.

In one embodiment, the heating medium comprises heated gases exhaustedfrom burner 30. A catalytic burner or electrical resistance burneroperates at elevated temperatures. Cooling air exhausted from anelectric burner or product gases exhausted from a catalytic burner areoften greater than about 100 degrees Celsius when the gases leaves thefuel processor. For many catalytic burners, depending on the fuel sourceemployed, the heating medium is commonly greater than about 200 degreesCelsius when the heating medium leaves the fuel processor. These heatedgases are transported to the fuel cell for convective heat transfer inthe fuel cell, such as passing the heated gases over one or more heattransfer appendages 46 for convective heat transfer from the warmergases into the cooler heat transfer appendages.

In another embodiment, burner 30 is a catalytic burner and the heatingmedium comprises the fuel source. Catalytic combustion in burner 30 isoften incomplete and the burner exhaust gases include unused and gaseousmethanol. Fuel cell 20 then comprises a thermal catalyst thatfacilitates production of heat in the fuel cell in the presence ofmethanol. The fuel source is typically vaporized prior to reaching theburner to facilitate catalytic combustion. In this case, line 35transports the gaseous and unused methanol to the thermal catalyst infuel cell 20. Several suitable thermal catalyst arrangements fortransferring heat into heat transfer appendages 46 are described below(FIG. 6A). Suitable methanol catalysts, such as platinum or palladiumcoated onto alumina pellets, are also described above with respect tocatalyst 104 in burner 30.

In one embodiment, the heating medium is transported to the fuel cellduring a start-up period before the fuel cell begins generatingelectrical energy, e.g., in response to a request for electrical energy.Heating a fuel cell in this manner allows fuel cell component operatingtemperatures to be reached sooner and expedites warm-up time needed wheninitially turning on fuel cell 20. In another embodiment, the heatingmedium is transported from the fuel processor to the fuel cell during aperiod of non-activity in which the fuel cell does not generateelectrical energy and the component cools. Since many fuel cells requireelevated temperatures for operation and the electrical energy generatingprocess is exothermic, the fuel cell usually does not require externalheating during electrical energy generation. However, when electricalenergy generation ceases for an extended time and the component dropsbelow a threshold operating temperature, the heating medium may then betransported from the fuel processor to regain the operating temperatureand resume electrical energy generation. This permits operatingtemperatures in a fuel cell to be maintained when electrical energy isnot being generated by the fuel cell.

A fuel cell package may include other fuel processor designs. Manyarchitectures employ a planar reformer disposed on top or below to aplanar burner. Micro-channel designs fabricated in silicon commonlyemploy such stacked planar architectures may be used. Other fuelprocessors may be used that process fuel sources other than methanol.Fuel sources other than methanol were listed above, and processors forthese fuels are not detailed herein for sake of brevity.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents thatfall within the scope of this invention which have been omitted forbrevity's sake. For example, although the present invention hasdescribed fluid delivery systems and methods operating in a fuel cellsystem and package, many of the methods and techniques described hereinare suitable for use with other micro-fluidics applications such asdelivery of fluids in medical applications, CPU cooling, ink delivery,electronics cooling and scientific applications required accuratecontrol over low fluid flow rates. Thus, fluid control according to thepresent invention is not limited to use in a fuel cell system. It istherefore intended that the scope of the invention should be determinedwith reference to the appended claims.

1. A method for controlling liquid delivery in a proton exchangemembrane fuel cell system, the method comprising: moving a liquid at aflow rate using a pressure source, wherein the flow rate is betweenabout 10 and about 200 milliliters per hour; flowing the liquid througha differential flow meter, wherein the differential flow meter isdisposed after the pressure source in a flow path and before receipt ofthe liquid by a fuel processor included in the fuel cell system, thedifferential flow meter including a flow restriction and at least onesensor that is configured to measure differential pressure in the liquidbefore and after the liquid passes the flow restriction, wherein theflow restriction includes an orifice with a smaller opening than a linethat supplies the liquid to the flow restriction; as the liquid flowspast the flow restriction in the differential flow meter, using the atleast one sensor of the differential flow meter to measure thedifferential pressure of the liquid before and after the liquid passesthe flow restriction; determining flow rate of the liquid using thedifferential pressure; and changing the flow rate of the liquid.
 2. Themethod of claim 1 wherein the liquid moves at a flow rate that is lessthan about 4 milliliters per hour per watt output by a fuel cellincluded in the fuel cell system.
 3. The method of claim 1 furthercomprising actuating a shut-off valve to block flow of the liquid from astorage device to the fuel processor when pressure in the storage devicerises when the fuel processor is off.
 4. The method of claim 1 furthercomprising a) detecting temperature of the liquid, and b) determiningflow rate of the liquid using detected temperature.
 5. The method ofclaim 1 wherein the pressure source is a pump and changing the flow rateof the liquid includes altering performance of the pump.
 6. The methodof claim 5 wherein the pump is a peristaltic or a piezo-actuated pump.7. The method of claim 1 wherein the pressure source applies a positivepressure to a bladder, in a fuel cartridge, that contains the fuel. 8.The method of claim 1 wherein the liquid includes methanol.
 9. Themethod of claim 8 wherein the liquid includes a mixture of methanol andwater.
 10. A method for controlling fuel provision in a proton exchangemembrane fuel cell system, the method comprising: moving a liquid fuelat a flow rate using a pressure source, wherein the flow rate of theliquid fuel is between about 10 and about 200 milliliters per hour;flowing the fuel through a differential flow meter, wherein thedifferential flow meter is disposed after the pressure source in a flowpath and before receipt of the fuel by a fuel processor included in thefuel cell system, the differential flow meter including a flowrestriction and at least one sensor, wherein the flow restrictionincludes an orifice with a smaller opening than a line that supplies theliquid to the flow restriction; as the fuel flows past the flowrestriction in the differential flow meter, using the at least onesensor of the differential flow meter to measure differential pressureof the fuel before and after the fuel passes the flow restriction;determining flow rate of the fuel using the differential pressure; andchanging the flow rate of the fuel.
 11. The method of claim 10 whereinthe fuel moves at a flow rate that is less than about 4 milliliters perhour per watt output by a fuel cell included in the fuel cell system.12. The method of claim 10 further comprising actuating a shut-off valveto block flow of the fuel from a storage device to the fuel processorwhen pressure in the storage device rises when the fuel processor isoff.
 13. The method of claim 10 further comprising a) detectingtemperature of the fuel, and b) determining flow rate of the fuel usingdetected temperature.
 14. The method of claim 10 wherein the pressuresource is a pump and changing the flow rate of the fuel includesaltering performance of the pump.
 15. The method of claim 14 wherein thepump is a peristaltic or a piezo-actuated pump.
 16. The method of claim10 wherein the pressure source applies a positive pressure to a bladder,in a fuel cartridge, that contains the fuel.
 17. The method of claim 10wherein the fuel includes methanol.
 18. The method of claim 17 whereinthe fuel includes a mixture of methanol and water.